Golf ball

ABSTRACT

A golf ball includes, as a constituent element thereof, a molded and crosslinked product obtained from a rubber composition containing (A) a base rubber which includes a polybutadiene having a cis-1,4 bond content of at least 60 wt %, (B) an unsaturated carboxylic acid and/or a metal salt thereof, (C) an organosulfur compound, (D) one or more organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides, and (E) one or more organic peroxide selected from the group consisting of hydroperoxides. In the golf ball, the value obtained by subtracting a center hardness of the molded and crosslinked product from a surface hardness thereof, expressed as the JIS-C hardness, is at least 20. The golf ball has a reduced spin rate on shots with a driver and therefore is able to follow a reliable trajectory that does not tend to dip or to rise in a high arc, enabling an increased distance to be achieved.

BACKGROUND OF THE INVENTION

The present invention relates to a solid golf ball. More specifically, the invention relates to a golf ball designed to reduce the spin rate and increase the distance traveled by the ball on shots with a driver.

Key performance features required in a golf ball include distance, controllability, durability and feel on impact. Balls having these qualities in the highest degree are constantly being sought. Among recent golf balls, a succession of balls having multilayer structures which are typically composed of three pieces have emerged. By having the structure of a golf ball be multilayered, it is possible to combine numerous materials of differing properties, thus enabling a wide variety of ball designs in which each layer has a particular function.

In particular, the solid core (sometimes referred to below simply as the “core”) located at the center of the golf ball generally accounts for most of the golf ball volume, and its properties exert a large influence on the performance of the ball as a whole. Hence, improving the core properties leads to significant enhancement in the ball performance.

In particular, JP-A 2001-259080 discloses an invention relating to a two-piece solid golf ball wherein, by setting the value obtained by subtracting the core center hardness from the core surface hardness to a JIS-C hardness of at least 20, the spin rate of the ball when struck with a driver is reduced and an increased distance is achieved. However, this prior-art invention has the drawback that sulfur is included within the rubber composition, as a result of which the crosslinking rate decreases, lowering the productivity.

For manufacturers that mass-produce golf balls, it is essential that all manufactured products have a good, uniform quality. Having the quality of the core be reliably good and uniform is of great importance. Accordingly, there exists a desire for a way to reliably set the difference between the core center and core surface hardnesses, expressed as the JIS-C hardness, to at least 20.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which is capable of reducing the spin rate on shots with a driver and which has an excellent flight performance.

The inventors have conducted extensive investigations in order to attain the above object. As a result, they have discovered that, when preparing a rubber composition for forming a one-piece solid golf ball or the core of a solid golf ball having a cover of one or more layer, in addition to including within the composition a specific polybutadiene-containing base rubber, an organosulfur compound, an unsaturated carboxylic acid and/or a metal salt thereof, and an organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides, by employing as a further additive an organic compound hydroperoxide—which has not hitherto been used as a rubber crosslinking agent, it is possible to easily and reliably obtain without loss of productivity a molded and crosslinked product for which the value calculated by subtracting the center hardness of the product from the surface hardness thereof, expressed as the JIS-C hardness, is at least 20. Moreover, the inventors have found that, in one-piece solid golf balls and solid golf balls having a cover of one or more layer that are fabricated using such a molded and crosslinked product, the spin rate on shots with a driver can be reduced, improving the flight performance.

Accordingly, the invention provides the following solid golf ball.

[1] A golf ball comprising a molded and crosslinked product obtained from a rubber composition comprised of components A to E below:

(A) a base rubber which includes a polybutadiene having a cis-1,4 bond content of at least 60 wt %;

(B) an unsaturated carboxylic acid and/or a metal salt thereof;

(C) an organosulfur compound;

(D) one or more organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides; and

(E) one or more organic peroxide selected from the group consisting of hydroperoxides,

wherein the value obtained by subtracting a center hardness of the molded and crosslinked product from a surface hardness thereof, expressed as the JIS-C hardness, is at least 20.

[2] The golf ball of [1], wherein the peroxyketal of component D is one or more selected from the group consisting of 1,1-di(t-hexylperoxy)cyclohexane, 1,1-di(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di(t-butylperoxy)-2-methylcyclohexane, 2,2-di(t-butyl-peroxy)butane, n-butyl-4,4-di(t-butylperoxy)valerate and 2,2-di(4,4-di(t-butylperoxy)cyclohexyl)propane. [3] The golf ball of [1], wherein the dialkylperoxide of component D is one or more selected from the group consisting of di(2-t-butylperoxyisopropyl)benzene, dicumyl peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, t-butylcumyl peroxide, di-t-hexyl peroxide, di-t-butyl peroxide and 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne. [4] The golf ball of [1], wherein the hydroperoxide of component E is one or more selected from the group consisting of p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide and t-butyl hydroperoxide. [5] The golf ball of [1], wherein the rubber composition includes, per 100 parts by weight of component A: from 10 to 60 parts by weight of component B, from 0.1 to 5 parts by weight of component C, from 0.25 to 8 parts by weight of component D, and from 0.05 to 5 parts by weight of component E.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing the golf balls of Examples 10 and 11 according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below. The golf balls of the present invention are one-piece solid golf balls formed using the subsequently described specific rubber composition, and solid golf balls having a cover of one or more layer formed over a solid core that has been formed using such a rubber composition.

The internal construction of the inventive golf ball may be suitably selected within a range that does not depart from the objects of the invention, insofar as use is made of a molded and crosslinked product which is obtained using the above rubber composition and which has a specific hardness difference. For example, in cases where a three-piece solid golf ball having a cover of two layers—an inner layer and an outer layer—is used, as shown in FIG. 1, the golf ball will have a three-layer construction that includes a core 1, an inner cover layer (intermediate layer) 2 encasing the core 1, and an outer cover layer 3 encasing the inner cover layer 2. When the ball has a multilayer construction with a cover of two or more layers, all the cover layers are sometimes referred collectively herein as “the cover.” Numerous dimples D are generally formed on the surface of the outer cover layer 3. These dimples D satisfy the subsequently described parameters of the present invention. FIG. 1 shows a three-piece solid golf ball G having a core 1, an inner cover layer 2 and an outer cover layer 3. However, as noted above, this construction may be suitably varied within a range that does not depart from the objects of the invention. For example, the ball may be one in which the above molded and crosslinked product itself is used as a one-piece solid golf ball (in which case, a large number of dimples D are formed on the surface of the molded and crosslinked product), one in which a single cover layer is formed to give a two-piece solid golf ball, or one in which three or more cover layers are formed to give a golf ball having a multi-piece construction of four or layers. In addition, the core 1 may be formed as a plurality of layers.

Here, the golf ball includes, as a constituent element thereof, a molded and crosslinked product of a rubber composition containing components A to E below:

(A) a base rubber which includes a polybutadiene having a cis-1,4 bond content of at least 60 wt %,

(B) an unsaturated carboxylic acid and/or a metal salt thereof,

(C) an organosulfur compound,

(D) one or more organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides, and

(E) one or more organic peroxide selected from the group consisting of hydroperoxides.

Polybutadiene may be suitably used as the base rubber of component A. In particular, it is recommended that use be made of a polybutadiene having a cis-1,4 bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4 bond content among the bonds on the molecule may result in a lower resilience.

Also, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of preferably not more than 20, more preferably not more than 1.70, and even more preferably not more than 1.5%. Too high a 1,2-vinyl bond content may result in a lower resilience.

Component A may include rubber ingredients other than the foregoing polybutadiene, within a range that does not detract from the advantageous effects of the invention. Examples of such rubber ingredients other than the above-described polybutadiene include polybutadienes other than the above polybutadiene, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

The unsaturated carboxylic acids and metal salts of unsaturated carboxylic acids of component B are included as co-crosslinking agents.

Examples of unsaturated carboxylic acids include, but are not limited to, acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

Examples of the metal salts of unsaturated carboxylic acids include, but are not limited to, the above-mentioned unsaturated carboxylic acids neutralized with a desired metal ion. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The amount of component B included per 100 parts by weight of the base rubber may be set to preferably at least 10 parts by weight, and more preferably at least 15 parts by weight. The upper limit in the amount included per 100 parts by weight of the base rubber may be set to preferably not more than 60 parts by weight, more preferably not more than 50 parts by weight, and even more preferably not more than 45 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.

The organosulfur compound of component C is included so as to enhance the rebound of the golf ball and increase the initial velocity of the ball. The organosulfur compound is not subject to any particular limitation, provided it is capable of improving the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. Illustrative examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol, and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The zinc salt of pentachlorothiophenol is especially preferred.

The amount of component C included per 100 parts by weight of the base rubber may be set to preferably at least 0.1 part by weight, and more preferably at least 0.4 part by weight. It is recommended that the upper limit in the amount of the organosulfur compound included per 100 parts by weight of the base rubber be preferably not more than 5 parts by weight, and more preferably not more than 3 parts by weight. If too little organosulfur compound is included, a sufficient rebound-improving effect may not be obtained. On the other hand, if too much organosulfur compound is included, further improvement in the rebound (especially when struck with a W#1) is unlikely to be achieved and the core may become too soft, possibly resulting in a poor feel.

Component D is an organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides, and is included in the rubber composition as a crosslinking agent.

First, illustrative examples of the peroxyketals include 1,1-di(t-hexylperoxy)cyclohexane, 1,1-di(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di(t-butylperoxy)-2-methylcyclohexane, 2,2-di(t-butylperoxy)butane, n-butyl-4,4-di(t-butylperoxy)valerate and 2,2-di(4,4-di(t-butylperoxy)cyclohexyl)propane. The use of 1,1-di(t-hexylperoxy)cyclohexane is especially preferred. These may be used singly or as a combination of two or more thereof. In addition, they may be included in combination with the subsequently described dialkyl peroxides.

Commercial products may be used as the peroxyketals. For example, suitable use may be made of Perhexa HC, Perhexa TMH, Perhexa C, Perhexa V, Perhexa 22, Perhexa MC and Pertetra A (available from NOF Corporation).

Next, illustrative examples of the dialkyl peroxides include di(2-t-butylperoxy isopropyl)benzene, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, t-butylcumyl peroxide, di-t-hexyl peroxide, di-t-butyl peroxide and 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne. The use of dicumyl peroxide is especially preferred. These may be used singly or as a combination of two or more thereof. In addition, they may be included in combination with the above-described peroxyketals.

Commercial products may be used as the dialkyl peroxides. For example, suitable use may be made of Percumyl D, Perhexa 25B, Perbutyl P (Peroxymon F-40), Perbutyl C, Perhexyl D, Perbutyl D and Perhexyne 25B (available from NOF Corporation).

The amount of component D included per 100 parts by weight of the base rubber may be set to preferably at least 0.25 part by weight, and more preferably at least 0.3 part by weight. The upper limit in the amount included per 100 parts by weight of the base rubber is preferably not more than 8 parts by weight, and more preferably not more than 5 parts by weight. If the amount included is too high, the molded and crosslinked product may have a poor durability. On the other hand, if the amount included is too low, a molded and crosslinked product of the desired hardness may not be attainable. When the above peroxyketal and the above dialkyl peroxide are used together, the combined amount thereof should be set within the above-indicated range, and the relative proportions in which both are included are not subject to any particular limitation.

Component E is one or more organic peroxide selected from among hydroperoxides and, as with above component D, is included as a crosslinking agent for the rubber composition. This component E has never before been used as a crosslinking agent for rubber. In the present invention, by including component E in combination with components C and D, the value obtained by subtracting the center hardness of the molded and crosslinked product from the surface hardness thereof can be set to a JIC-C hardness of at least 20.

Illustrative examples of the hydroperoxide include p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, 1,1,3,3,-tetramethylbutyl hydroperoxide, cumene hydroperoxide and t-butyl hydroperoxide. The use of p-menthane hydroperoxide and diisopropylbenzene hydroperoxide is especially preferred. These may be used singly or as a combination of two or more thereof.

Commercial products may be used as the hydroperoxide. For example, suitable use may be made of Perbutyl H, Percumyl H, Percumyl P, Permenta H or Perocta H (all available from NOF Corporation).

The amount of component E included per 100 parts by weight of the base rubber may be set to preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit in the amount included per 100 parts by weight of the base rubber is preferably not more than 5 parts by weight, and more preferably not more than 3 parts by weight. If the amount included is too high, effects which correspond to the amount included may not be achievable. On the other hand, if the amount included is too low, the desired effects may not be achievable.

Various additives may be optionally included in the rubber composition. For example, additives such as sulfur, inert fillers, antioxidants and zinc stearate may be included.

Examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.

The amount of inert filler included per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, and more preferably at least 5 parts by weight. The upper limit in the amount included per 100 parts by weight of the base rubber may be set to preferably not more than 200 parts by weight, more preferably not more than 150 parts by weight, and even more preferably not more than 100 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.

The antioxidant used may be a known antioxidant and is not subject to any particular limitation. Specific examples include commercial products such Nocrac NS-6, Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.

The amount of antioxidant included may be set to more than 0, and is set to preferably at least 0.02 part by weight, and more preferably at least 0.05 part by weight, per 100 parts by weight of the base rubber. The upper limit, although not subject to any particular limitation, may be set to preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, even more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight, per 100 parts by weight of the base rubber. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.

The above rubber composition can be obtained by blending the various above ingredients using a conventional mixing apparatus (e.g., a Banbury mixer, kneader, roll mill). When this rubber composition is used to fabricate a one-piece solid golf ball or a solid golf ball having a cover of one or more layer, a conventional molding method, such as injection molding or compression molding, may be employed. In such a case, conventional conditions may be employed as the vulcanization conditions, which may be suitably set in accordance with the size, deflection and other characteristics of the molded and crosslinked product. Vulcanization is generally a one-step process, although it may be carried out in two separate stages (two-step vulcanization) and is not subject to any particular limitation.

In this invention, the diameter of the molded and crosslinked product (a one-piece solid ball, or the core of a solid golf ball having a cover of one or more layer) formed using the above rubber composition, although not subject to any particular limitation, is set as appropriate for the ball construction.

With regard to the hardness of the molded and crosslinked product, it is essential for the hardness difference between the center hardness and the surface hardness to be specified. Specifically, the value obtained by subtracting the center hardness of the crosslinked and molded product from the surface hardness thereof, expressed as the JIS-C hardness, must be at least 20. If this hardness difference is low, the spin rate becomes too high, lowering the distance performance. The upper limit in the hardness difference is preferably set to not more than 40, and more preferably not more than 35. If this hardness difference is too large, the rebound of the golf ball as a whole may tend to decrease, in addition to which the durability to repeated impact may worsen.

The center hardness of the molded and crosslinked product is not subject to any particular limitation, although it is recommended that this value, expressed as the JIS-C hardness, be preferably at least 40, and more preferably at least 43. The upper limit of this value is not subject to any particular limitation, although it is recommended that the upper limit be preferably not more than 80, and more preferably not more than 75.

The surface hardness of the molded and crosslinked product is not subject to any particular limitation, although it is recommended that this value, expressed as the JIS-C hardness, be preferably at least 60, and more preferably at least 63. The upper limit of this value is not subject to any particular limitation, although it is recommended that the upper limit be preferably not more than 95, and more preferably not more than 93.

The molded and crosslinked product has a deflection when subjected to loading, i.e., a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which, although not subject to any particular limitation, may be set to preferably at least 2 mm, and more preferably at least 2.5 mm. The upper limit also is not subject to any particular limitation, although it is recommended that the upper limit be preferably not more than 6 mm, and more preferably not more than 5.8 mm. If the deflection is low, the molded and crosslinked product may be too hard and the ball may have a harder feel on impact, in addition to which the spin rate on shots with a driver may increase, shortening the distance traveled by the ball. On the other hand, if the deflection is too large, a sufficient rebound may not be attainable, which may result in a shorter distance.

When a cover is formed on the above molded and crosslinked product so as to obtain a solid golf ball having a cover of one or more layer, the cover may be formed of a known material. More specifically, use may be made of an ionomeric resin, a polyester-type thermoplastic elastomer, a polyamide-type thermoplastic elastomer, a polyurethane-type thermoplastic elastomer, an olefin-type thermoplastic elastomer, and mixtures thereof. Commercial products may be used as these materials. Illustrative examples of such commercial products include Himilan (ionomeric resins produced by DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn (ionomeric resins produced by E.I. DuPont de Nemours & Co.) and Iotek (ionomeric resins produced by ExxonMobil Chemical).

Various additives, such as UV absorbers, antioxidants, metal soaps, pigments and inorganic fillers, may be included in suitable amounts in the above cover material.

The cover may be formed by a known process. Use may be made of, e.g., injection molding or compression molding. For example, in cases where the cover is formed by injection molding, a solid core fabricated beforehand from the above rubber composition may be set within a cover-forming mold and the cover material injected into the mold according to a conventional method. In another approach that may be used, a pair of half-cups is pre-molded using the above cover material, the core is enclosed with these half-cups, and compression molding is carried out at, for example, between 120 and 170° C. for a period of 1 to 5 minutes.

In cases where a cover is formed in this invention, the thickness is not subject to any particular limitation, but may be set to preferably at least 0.2 mm, and more preferably at least 0.4 mm. The cover thickness is not subject to any particular upper limit. In cases where the cover is composed of a plurality of (two or more) layers, it is desirable for the combined thickness of all the cover layers to fall within the above-indicated range.

In this invention, the deflection by the golf ball on which a cover has been formed, although not subject to any particular limitation, may be set to preferably at least 2 mm, and more preferably at least 2.2 mm. It is recommended that the upper limit in the deflection, although not subject to any particular limitation, be set to not more than 6 mm, and preferably not more than 5.5 mm. In cases where no cover is formed and the molded and crosslinked product is used as a one-piece solid golf ball, although not subject to any particular limitation, it is recommended that the deflection of the golf ball be the same as the deflection in the above-described golf balls having a cover formed thereon.

In the golf ball of the invention, although not subject to any particular limitation, to further improve the aerodynamic properties and increase the distance traveled by the ball, as in conventional golf balls, a plurality of dimples may be formed on the surface. By optimizing dimple parameters such as the types and total number of dimples, the trajectory of the ball can be further stabilized, enabling a ball having an excellent flight performance to be obtained. Moreover, to enhance the design and durability of the golf ball, various treatment, such as surface preparation, stamping and painting, may be carried out on the surface of a one-piece solid golf ball or on the surface of the cover of a solid golf ball having one or more cover layer.

In the golf ball of the invention, which may be manufactured so as to conform with the Rules of Golf for competitive play, the ball diameter is preferably set to not less than 42.67 mm and the ball weight is preferably set to not more than 45.93 g.

The golf ball of the invention has a reduced spin rate on shots with a driver and can therefore follow a reliable trajectory which does not tend to dip or rise in a high arc, enabling an increased distance to be achieved.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 9, Comparative Examples 1 to 7 Formation of Core

Rubber compositions formulated as shown in Table 1 were prepared, then molded and vulcanized at 155° C. for 20 minutes to form cores having a diameter of 37.3 mm.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 Formulation Poly- 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 (pbw) butadiene Zinc acrylate 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 Organosulfur 1 1 1 1 1 1 0.5 1 1 1 1 0.1 1 compound Inorganic 0.5 sulfur compound Zinc oxide 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Peroxyketal 1.2 1.2 1.2 3 1.2 1.2 3 1.2 1.2 1.2 1.2 1.2 1.2 0.5 Dialkyl 3 3 3 peroxide Hydro- 1 0.5 1 2 1 1 peroxide (1) Hydro- 1 1 1 1 1 1 1 1 peroxide (2) Hardness Center 66 56 55 50 46 57 50 47 51 69 52 67 62 64 56 58 57 (JIS-C) hardness Surface 86 83 76 75 67 81 71 74 81 87 71 83 67 74 71 74 68 hardness Surface 20 27 21 25 21 24 21 27 30 18 19 16 5 10 15 16 11 hardness − Center hardness Properties Deflection (mm) 2.8 2.7 4.0 4.1 5.5 3.8 4.7 4.2 3.7 2.4 3.9 3.1 3.7 3.7 4.3 3.9 4.7

Details of the materials mentioned in Table 1 are given below.

-   Polybutadiene: Available under the trade name “BR 730” from JSR     Corporation -   Zinc acrylate: Composed of 85 wt % zinc acrylate and 15 wt % stearic     acid -   Organosulfur compound:     -   Zinc salt of pentachlorothiophenol (ZnPCTP) -   Inorganic sulfur compound: Manganese sulfate -   Zinc oxide: Grade 3 ZnO -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-t-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Peroxyketal: 1,1-Di(t-butylperoxy)cyclohexane, available under the     trade name “Perhexa C-40” from NOF Corporation -   Dialkyl peroxide: Dicumyl peroxide, available under the trade name     “Percumyl D” from NOF Corporation -   Hydroperoxide (1): p-Menthane hydroperoxide, available under the     trade name “Permenta H” from NOF Corporation -   Hydroperoxide (2): Diisopropylbenzene hydroperoxide, available under     the trade name “Percumyl P” from NOF Corporation

Various properties, such as the JIS-C hardnesses in different areas and the deflection, were evaluated by the subsequently described test methods for each of the cores obtained as described above. The results are shown together in Table 1. These properties were all measured in a 23° C. environment. Moreover, because it is apparent from subsequently described Examples 10 and 11 that a decrease in the spin rate can be achieved by having the hardness difference between the center hardness and the surface hardness of the core satisfy the value specified in this application, it suffices here for the hardness difference between the center hardness and the surface hardness of the core to be measured.

Examples 10 and 11, Comparative Examples 9 and 10 Formation of Core

Rubber compositions formulated as shown in Table 2 were prepared, then molded and vulcanized at 155° C. for 20 minutes to form cores having a diameter of 37.3 mm. The materials mentioned in Table 2 were the same as those mentioned in Table 1 above.

TABLE 2 Formulation Example Comparative Example (pbw) 10 11 9 10 Polybutadiene 100 100 100 100 Zinc oxide 21.2 21.3 23 22.9 Antioxidant 0.1 0.1 0.1 0.1 Organosulfur compound 1 1 Zinc acrylate 36.25 36.25 28.75 33 Peroxyketal 1.2 1.2 1.2 1.2 Hydroperoxide (1) 1 1 Hydroperoxide (2) 1 Blending ratio 1.170 1.171 1.161 1.169

Formation of Intermediate Layer and Cover

Next, an intermediate layer-forming material of the formulation shown below was injection-molded over the core obtained as described above so as to form an intermediate layer, then a cover-forming material of the formulation shown below was injection-molded over the intermediate layer so as to form a cover, thereby giving a three-piece solid golf ball. To obtain the cover-forming material, first, polyurethanes (i-1) and (i-2) were uniformly mixed and pelletized to prepare material (i) and an isocyanate mixture was pelletized to prepare material (ii), following which materials (i) and (ii) were dry-blended and fed to an injection molding machine.

Formulation of Intermediate Layer-Forming Material:

Himilan 1605 70 parts by weight Dynaron 6100P 30 parts by weight Behenic acid 20 parts by weight Polytail H 2 parts by weight Calcium hydroxide 3 parts by weight

Details of the above materials are given below.

-   Himilan 1605: An ionomer resin available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Dynaron 6100P: A hydrogenated polymer available from JSR

Corporation

-   Behenic acid: Available from NOF Corporation -   Polytail H: A low-molecular-weight polyolefin polyol available from     Mitsubishi Chemical Corporation -   Calcium hydroxide: Available from Shiraishi Kogyo

Formulation of Cover-Forming Material:

(i) Polyurethane (i-1) Pandex T8295 75 parts by weight (i-2) Pandex T8290 25 parts by weight (ii) Isocyanate mixture 20 parts by weight

Details of the above materials are given below.

-   Pandex T8295: An MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer. Resin hardness, (JIS-A) 97;     rebound resilience, 44%. -   Pandex T8290: An MDI-PTMG type thermoplastic polyurethane material     available from DIC Bayer Polymer. Resin hardness (JIS-A), 93;     rebound resilience, 52%. -   Isocyanate mixture: A masterbatch containing 30 wt % of     4,4′-diphenylmethane diisocyanate (masterbatch base resin: polyester     elastomer) available under the trade name “Crossnate EM-30” from     Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.

Each of the golf balls obtained was tested and evaluated by the methods described below with regard to properties of the various layers, such as thickness, hardness and deflection, and also flight performance. The results are shown in Table 3. These properties were all measured in a 23° C. atmosphere.

(1) Core or Ball Deflection (mm)

The core or ball was placed on a hard plate, and the amount of deformation when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

(2) Core Surface Hardness

The core surface is spherical. The durometer indenter was set substantially perpendicular to the spherical surface of the core, and JIS-C hardness measurements (in accordance with JIS-K6301) were taken at two randomly selected points on the core surface. The average of the two measurements was used as the core surface hardness.

(3) Core Center Hardness

The core was cut into half, creating a flat plane. The durometer indenter was set substantially perpendicular at the center thereof, and the JIS-C hardness was measured (in accordance with JIS-K6301).

(4) Flight Performance of Ball

The distance traveled by the ball when hit at a head speed (HS) of 50 m/s with a driver (abbreviated below as “W#1”; TourStage X-Drive 701, manufactured by Bridgestone Sports Co., Ltd.; shaft flex, X; loft angle, 9.5°) mounted on a golf swing robot was measured. In addition, the spin rate was measured, using an apparatus for measuring initial conditions, immediately after hitting the ball in the same way as just described.

TABLE 3 Comparative Example Example 10 11 9 10 Core Diameter (mm) 37.3 37.3 37.3 37.3 Deflection (mm) 3.6 3.7 3.5 3.5 Center hardness, JIS-C 56 54 63 62 Surface hardness, JIS-C 83 83 82 73 Hardness difference 27 29 19 11 Intermediate layer Thickness (mm) 1.7 1.7 1.7 1.7 Cover Thickness (mm) 1.0 1.0 1.0 1.0 Ball Diameter (mm) 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.4 45.5 Deflection (mm) 2.8 2.8 2.7 2.9 Flight, W#1 Carry (m) 244 244 240 238 (HS, 50 m/s) Spin rate (rpm) 2,840 2,840 2,900 2,880 

1. A golf ball comprising a molded and crosslinked product obtained from a rubber composition comprised of components A to E below: (A) a base rubber which includes a polybutadiene having a cis-1,4 bond content of at least 60 wt %; (B) an unsaturated carboxylic acid and/or a metal salt thereof; (C) an organosulfur compound; (D) one or more organic peroxide selected from the group consisting of peroxyketals and/or dialkyl peroxides; and (E) one or more organic peroxide selected from the group consisting of hydroperoxides, wherein the value obtained by subtracting a center hardness of the molded and crosslinked product from a surface hardness thereof, expressed as the JIS-C hardness, is at least
 20. 2. The golf ball of claim 1, wherein the peroxyketal of component D is one or more selected from the group consisting of 1,1-di(t-hexylperoxy)cyclohexane, 1,1-di(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di(t-butylperoxy)-2-methylcyclohexane, 2,2-di(t-butylperoxy)butane, n-butyl-4,4-di(t-butylperoxy)valerate and 2,2-di(4,4-di(t-butylperoxy)cyclohexyl)propane.
 3. The golf ball of claim 1, wherein the dialkylperoxide of component D is one or more selected from the group consisting of di(2-t-butylperoxyisopropyl)benzene, dicumyl peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane, t-butylcumyl peroxide, di-t-hexyl peroxide, di-t-butyl peroxide and 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne.
 4. The golf ball of claim 1, wherein the hydroperoxide of component E is one or more selected from the group consisting of p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide and t-butyl hydroperoxide.
 5. The golf ball of claim 1, wherein the rubber composition includes, per 100 parts by weight of component A: from 10 to 60 parts by weight of component B, from 0.1 to 5 parts by weight of component C, from 0.25 to 8 parts by weight of component D, and from 0.05 to 5 parts by weight of component E. 